Unlocking is a common term used to describe when a merchant takes an item to the next predetermined level of improvement, referred to as a Tier. This usually means the item will have more verb traps available to the owner. In rare cases, as with Voln armor and Ithziri armor, this means that the item will have more mechanical benefits such as padding, a boost to sheer fear resistance, or other special ability. Items that can be unlocked are often sold at festivals and pay events; they are not typically available in any town shop.
Use of ANALYZE will usually let one know whether an item can be unlocked. This is not a universal rule, especially for older items. Also, sometimes "Tier 1" is considered off-the-shelf (not unlocked), while with other items "Tier 0" will be considered off-the-shelf. Even more confusing, Tier 0 might mean that the item has been unlocked once, or the special script has been applied (Ithziri armor).
In the case of Ithziri armor, and some other rare scripts, no item is ever available for purchase off-the-shelf.